Conventionally, there are various proposals related to an automatic focusing technique used in a still camera, a TV camera, and a video camera.
For example, contrast based automatic focusing (hereinafter “contrast AF”) is proposed, in which high frequency components of the image picked up by a picked up image element is extracted and the focus position is controlled by maximizing an evaluation value corresponding to the sharpness of the image to perform focusing. In the contrast AF, a search operation, that is, changing the position of the focus lens, is required to determine the direction of the movement of the focus lens where the evaluation value corresponding to the contrast of the image relative to the focus position is the maximum and to determine the maximum point. It is known that the focusing accuracy of the contrast AF is high, because focusing is determined using an image signal obtained based on an output signal from the image pickup element that picks up an image of an object.
So-called external measuring AF is also proposed, in which a distance measuring sensor is arranged independently from an image pickup lens, and the in-focus position of the focus lens is calculated from an obtained object distance to control the focus lens to perform focusing. Phase difference system automatic focusing (hereinafter, “phase difference AF”) is also proposed, in which a split optical system splits a beam passed through an image pickup lens, an in-focus state with respect to the object is detected from the split beams, and focusing is performed based on the result.
Among these, in the external measuring AF and the phase difference AF, the beam from the object is divided into two beams in an in-focus state detecting apparatus to form two images on sensors. Correlation computing is applied to photoelectrically converted signals of two images to detect the phase difference. A defocus amount and an object distance are calculated from the phase difference, and the defocus amount and the object distance are converted to a drive target position of the focus lens to control the focus lens to perform focusing.
In the external measuring AF and the phase difference AF, it is known that there is no need to perform a search operation performed in the contrast AF, and the focusing speed is fast.
To utilize the high focusing accuracy of the contrast AF and the high focusing speed of the external measuring AF and the phase difference AF, a hybrid AF system in which a combination of the contrast AF and external measuring AF and phase difference AF is further proposed.
To maintain the in-focus state with respect to the target object by automatic focusing, an automatic focusing operation needs to be executed every time the object distance changes to cause the object being out of focus.
In the automatic focusing apparatus including only the contrast AF, if the evaluation value of the contrast AF for an object for which a focusing operation was once performed to obtain an in-focus state, decreases by a value greater than a predetermined value, an automatic focusing operation is executed again (hereinafter, focusing operation executed again after focusing will be described as “re-execution operation”).
In this case, if the object moves without involving a change in the object distance, such as when the object moves in a parallel direction (horizontal direction) relative to the imaging plane or when panning operation is performed, the evaluation value of the contrast AF may reduces when the object is moving while the object is in in-focus state.
Therefore, based only on the condition that the contrast AF evaluation value reduces by more than a predetermined value, it is difficult to determine whether the object distance is changed or the object has moved without involving a change in the object distance. The re-execution operation of AF that would not be necessary if the object is in in-focus state is executed, and it is significantly uncomfortable to view the picked up image if a search operation is performed.
For example, Japanese Patent Application Laid-Open No. 2007-174521 discloses a technique of using a blur detector to detect a change in the composition and controlling not to perform the focus adjustment during the change in the composition. When there is a change in the composition during shooting of a video, such as by panning, unstable focusing caused by the AF following the change in the distance status of the object in the screen can be prevented.
In the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2007-174521, the AF is terminated during the detection of the change in the composition. Therefore, unstable focusing can be prevented as described above when there is panning.
However, when the object moves parallel to the imaging plane instead of the change in the composition, the AF search may be performed and the focus may become unstable if the contrast AF evaluation value reduces during the movement of the object, because the change in the composition is not detected.
An object of the present invention is to provide an automatic focusing apparatus that can attain both the stability and the followability in AF by appropriately executing an AF re-execution operation when there is a movement of an object or a change in a composition.